Sinorhizobium medicae genes whose regulation involves the ActS and/or ActR signal transduction proteins.

ActS-ActR proteins belong to a highly conserved family of two-component signal transduction systems involved in global regulation in the alpha-proteobacteria; they were first identified in Sinorhizobium medicae (previously Sinorhizobium meliloti) as essential for acid-tolerance. This paper reports on the identification of genes regulated by ActS and/or ActR in S. medicae. To do this, random gusA fusions were created in S. medicae to follow gene transcription in an actS chromosomal knockout mutant containing plasmid-borne actS. Plasmid borne actS was cured from the mutants and beta-glucuronidase (GUS) activity compared between the different genetic backgrounds. We detected actS-dependent regulation of the genes gst1 (detoxification), hyuA (hydantoin utilization) and fixN2 (microaerobic respiration). We show that ActR is involved in regulating cbbS (CO2 fixation), narB (nitrate assimilation) and required for low pH and microaerobic induction of the nitrogen fixation regulators fixK and nifA. In particular, we demonstrate that the transcriptional activation of fixN2 is regulated by ActR through FixK.